Some important petroleum crude feedstocks, residua, and deasphalted oil derived from them, contain levels of iron which render them difficult to process effectively using conventional refining techniques. Among those iron contaminants causing particular problems are those found in the form of nonporphyrin, organometallically-bound compounds. These species have been attributed to either iron complexes naturally present in the crude or to solubilized iron produced from corrosion and decay of iron-bearing equipment which comes in contact with crude oils. One possible class of iron-containing compounds identified in particular is the respective naphthenates and their homologous series. These organometallic compounds are not separated from the feedstock by normal processes, and in a conventional refining technique they can cause the very rapid deactivation of hydroprocessing catalysts, in addition to plugging and heat transfer fouling of furnace and heat exchange tubes. Examples of feedstocks demonstrating objectionably high levels of iron include those from the San Joaquin Valley in Calif., generally contained in a pipeline mixture referred to as San Joaquin Valley crude or residuum.
The iron contaminants in these crudes can cause significant deleterious effects in various catalytic processes to which the feed is subjected. In particular, metals can seriously deactivate the catalysts used in these processes, as well as causing serious plugging of fixed or other type catalyst beds due to the deposition of iron sulfides in the interstitial space between the catalyst particles in the bed. Furthermore, precipitation of iron sulfides on heat exchanger and furnace tubes can cause plugging problems and heat transfer loss. Heat transfer loss can force operations of a catalyst bed at higher temperatures, thereby accelerating catalyst deactivation even further.
One known method for substantially reducing the metals in the feed to a hydrocracker uses a guard bed in a first reaction zone ahead of the the hydrocracker reactor, U.S. Pat. No. 3,365,389, Spars et al. According to the patent, the first reaction zone is maintained at a temperature of above about 820.degree. F. and at a pressure of between 1,000 and 5,000 psig. This has the disadvantage of requiring high temperatures and pressures, as well as the relatively high pressure drop through the guard bed after metals are preferentially deposited in the upper part of the bed. Furthermore, when these catalyst particles are spent, the feed metals concentrations across individual pellets are observed to be high at the outside and low toward the inside. Thus these catalysts are not efficiently used; some reactor volume is wasted, principally that occupied by the interiors of the catalyst particles. Moreover, these catalysts are expensive, typically costing several dollars per pound.
Prior art processes often also require high temperature and high hydrogen pressure, each of which introduces its own process difficulties. For example, U.S. Pat. No. 3,573,201 teaches a process for the removal of iron using contact particles such as hydrogenation catalysts or inorganic oxides. The process is conducted at both high temperatures (600.degree. F. and above), high pressures (200 to 5000 psig) and in the presence of hydrogen.
It is desirable then to provide a simple method for removing these organic iron compounds, without the process limitations of high temperature and high hydrogen pressure.